Made handle function private, and renamed to _handle in order to start driving by tests the new handle function. Once we are done, the new handle function will replace the old _handle one.
Implemented the EmailData case of the handle function, using the signature discussed at https://github.com/GreanTech/Qaiain/pull/22#discussion_r14571723.
Handled the case where an unknown message is received.
Handled the case where a large message is received.
Deleted large messages once they are handled.
Replaced the old handle function.
Added a test case covering the scenario where a message points to a non-existent BLOB, indicating that the message was already handled.